1. Field of the Invention
This invention relates to a multiwindow control system for use in information processing apparatus, such as workstations and computers, that are capable of displaying image information in multiple windows on a display screen. More particularly, this invention relates to a multiwindow control system for controlling displays having a plurality of windows that overlap with one another.
2. Discussion of the Related Art
With the expansion of use of information processing apparatus, such as workstations, the use of multiwindow systems in these apparatus has become common. The term "multiwindow system" means a system that is designed to display a plurality of windows on the same display screen. Using a multiwindow system is greatly advantageous because plural types of processing can be executed in the respective windows on the same screen and images can be copied using the plurality of windows.
One or more window systems may be used to display a plurality of windows on the display screen. If a single window system is used, a plurality of windows are displayed by one processing program. When more than one window system is used, a plurality of processing programs are responsible for displaying their own windows.
FIG. 11 shows an example of a window display in a multiwindow system. Display screen 11 shows windows 12A and 12B that are displayed by a first window system and windows 13A and 13B that are displayed by a second window system. Window 12A, which is the same size as the outside dimensions of screen 11, should serve as a parent when parentage is established by the first window system. This window is usually referred to as a "root" window. Assuming that a child window always exists within a parent, window 12B, which is displayed by the first window system, is a child of window 12A.
Windows 13A and 13B are established by the second window system within window 12B. The first and second window systems are different, and therefore, different capabilities of processing are usually performed depending upon whether a cursor (not shown) is located in the inside or outside of a window in the second window system. Windows 13A and 13B displayed by the second window system have no parentage since neither contains the other.
There is an overlap between windows in FIG. 11. Such an overlap between windows will frequently occur even in the same window system. This is shown in FIG. 12. Display screen 21 shows first window 22A which is overlaid with second window 22B and third window 22C. As far as first window 22A is concerned, the window contents are only displayed in two notched rectangular areas 23 and 24. The other areas are covered by either second window 22B or third window 22C, and no contents are displayed.
As will be readily understood from the foregoing description, the area of a window displayed by a multiwindow system but covered with another window cannot be seen. Of course, the covered area will become visible if the overlay window is either moved to another location, closed or reduced in size. Various techniques have been developed in the prior art to render the covered area visible. The uncovered area, which has become visible, can contain associated image information to be displayed on the screen.
On the other hand, the contents of the area of windows which remain visible despite overlapping are generally displayed by a displaying means provided by the window system. However, it is necessary to directly write image information in a display memory when high speed displaying. If the image information is written over the visible area, however, the image information in the visible area overflows and becomes incomplete.
In a multiwindow system, first and second window systems simultaneously write the image information into the display memory corresponding to the visible image area, thus causing overlapping portions. Additionally, an order of the overlapping is not fixed because display requests occur asynchronously according to the programs using the window systems. Accordingly, the contents of the display are varied corresponding to the order of the display requests.
In order to eliminate this problem, a display section of the second window system may be modified so that the means for writing the image information into the display memory actually utilizes the means provided by the first window system. Under these circumstances, however, processing by the display section of the second window system and processing by the first window system are executed simultaneously. Thus, a considerable amount of time is required for display control.